This invention relates to a debris detector and more particularly to such a detector for use in a particle counter such as a blood cell counter, for detecting and providing an alarm signal whenever debris becomes lodges against or in the orifice through which the particles pass.
Blood cell counters of the general type now known as the COULTER COUNTER (R) analyzer were first described in the U.S. Pat. No. 2,656,508 which was granted Oct. 20, 1953 in the name of Wallace H. Coulter. According to the principle taught by Coulter in the aforementioned patent, a fluid containing blood cells suspended therein, such as diluted whole blood, is passed through a small opening, or orifice, from one fluid containing chamber to another fluid containing chamber. An electrode positioned in each of the chambers is coupled to a current source, so that a constant current flows through the orifice from one chamber to the other. As blood cells pass through the orifice, the electrical resistance within the orifice increases with a corresponding increase in the voltage across the orifice due to the constant current. Sensing means coupled to the two electrons sense the voltage pulse increase due to the package of a particle through the orifice, thereby detecting such particle. By utilizing appropriate counter means and volume control means, the number of particles, such as red blood cells, within a defined volume, can be determined. The change in resistance in the orifice due to the presence of a particle is approximately proportional to the volume of the particle. Thus, the COULTER COUNTER analyzer is capable of determining the mean particle volume of, as well as the number of particles in, a sample being measured.
One of the problems which always has been present in any blood cell counter of the type described above is that debris, such as blood clots or other contaminants placed in the specimen under test from the ambient, can lodge against or within the orifice, thereby preventing the free and natural flow of the cells through the orifice. Other debris within the specimen may be so small that it passes through the orifice, causing no permanent problem. Such small debris is referred to herein as transient debris and sufficient care can be taken so that the number of transient debris particles passing through the orifice is negligible compared to desired particles. Thus, the transient debris, if counted as particles, causes negligible effect on the final result.
Many attempts have been made in the past to provide a detector to indicate that the orifice of the COULTER (R) type particle counter has become clogged by debris lodged in or against the orifice. For example, in U.S. Pat. No. 3,259,842 in the name of Wallace H. Coulter et. al., a debrils detector is described which measures the height and width of the pulses detected by the COULTER particle detector apparatus. This debris detector makes use of the fact that debris generally has a resistance greater than the resistance of a normal blood cell. Further, the duration of the pulse detected for a debris particle or lodged debris generally exceeds the duration of a normal cell. This is because a cell passes through the orifice in less time than debris, because the cell is smaller.
A second type debris detector is disclosed in U.S. Pat. No. 3,259,891, in the name of Wallace H. Coulter et. al., and utilizes similar larger resistance and pulse width for debris versus predictably known lower resistance and pulse time for normal particles passing through the orifice. In the debris alarm detector of U.S. Pat. No. 3,259,891 the magnitude of the voltage sensed is first detected and, for pulses exceeding a certain threshold voltage amplitude, the duration of the pulse is also measured. If the duration exceeds a certain value, then the debris alarm is sounded. The problem with this type of detection is that transient debris and many large particles are detected as debris, merely because of their large size and long duration their pulse.
Several problems exist with the technique in the debris alarm described in U.S. Pat. No. 3,259,891. First, the debris alarm of the prior art is unable to differentiate easily between debris which lodges in or against the orifice and large cells or transient debris which passes through the orifice causing negligible effect on the ultimate result. Further, in the prior art device the waveform shape becomes critical in detecting the debris. This is particularly true of the rise time of the pulse which results when debris becomes lodged in or against the orifice. In order to compensate for the relatively slow rise time of the leading edge of the debris pulse, the prior art device used a lower threshold voltage setting than was optimally desirable. This, in turn, resulted in many false alarms of debris being detected, particularly when the debris was transient debris which passed through the orifice. Further, the prior art device had difficulty in differentiating in many instances between large pulses due to large cells or debris, again leading to many false negatives alarms.
Attempts to overcome many of the limitations of the prior art debris detectors described in the aforementioned patent have been made. For example, in U.S. Pat. No. 4,412,175 in the name of Franklin D. Maynarez, bad pulses are detected based on the techniques of the aforementioned prior art patents to W. H. Coulter, et. al. and ratioing techniques are utilized to determine when the number of bad pulses exceeds certain predetermined unacceptable levels. By utilizing the improvements such as in the Maynarez patent, some of the shortcomings of the prior art detectors can be overcome. Another example of improvements in debris alarms of the prior art is shown in U.S. Pat. No. 4,450,435 in the name of Bobby D. James. In the James Patent, sophisticated comparator means are utilized for comparing the number of good pulses and bad pulses and for alarming when the number of bad pulses exceeds certain predetermined relative per cents. Even with these techniques, false negatives are obtained when the sample under test contains many large cells.
Rather than relying on ratioing and group comparison techniques of the prior art, it would be preferable to be able to utilize the debris alarm circuitry itself directly to provide an indication when debris becomes lodged in the particle counter orifice. This will result in an alarm when troublesome debris is detected, rather than requiring a wait until after the proper ratios or comparisons have been made. Such a wait causes a substantial number of improper particles having been counted. Further, by detecting the debris when it initially becomes lodged, samples may be saved and evasive action taken immediately.